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Cozy-client Architecture

This is an overview of the cozy-client architecture:

Architecture schema

To explain the architecture, we will see how a request for data flows through cozy-client through links and finally how the data that has been fetched is stored in the redux store.

Query definitions

To query data, we create a query definition: an object describing what documents to fetch.

  doctype: "io.cozy.todos",
  selector: {
    "finished": true

It is typically created via the helper Q that provides a fluid interface to create a Query Definition.

const qdef = Q('io.cozy.todos').where({ finished: true })

Then we need to execute this query, fetch the data, and storing it.

await client.query(qdef)

When executed, Query definitions are passed down to Links.

Links accept query definitions and can choose

  • either to return a response
  • or to pass the query definition down to the next link.

ℹ️ This is how offline support is implemented: on mobile, a PouchLink (we use PouchDB under the hood) is added as a link to cozy-client, before the default StackLink. The PouchLink can decide:

  • either to pass down the query definition to the next link (if the PouchLink has not been synchronized for example),
  • or to respond to the request by communicating directly with PouchDB.

At the moment there are only two links:

  • StackLink : fetches data over HTTP by communicating to the Cozy’s Stack via the StackClient.
  • PouchLink : fetches data from a local PouchDB. Useful to have offline-ready applications.


When links have responded with data, the data is stored inside the redux store that is internal to cozy-client. This redux store brings observability to cozy-client, and allows for connection of UI components to the data.

ℹ️ You do not need to use Redux in your application to use cozy-client.

ℹ️ You can connect your own store to cozy-client. It is useful for more advanced techniques where you create selectors directly on the data of cozy-client.

The redux store is composed of two slices, “documents” and “queries”:

  • documents stores the data indexed by [doctype]([doctype]( then _id.
  • queries store information for each query that has been done by cozy-client.
    • ids of the documents that match the query
    • whether the server has more documents that can be fetched (useful for pagination)
    • whether the query is being loaded.
    documents: {
        'io.cozy.todos': {
            'todo-id-1': {
                finished: true,
                label: "Add architecture document for cozy-client"
    queries: {
        finishedTodos: {
            data: ['todo-id-1']


ℹ️ If queries are not named, a name is automatically generated but this means that the queries slice can grow indefinitely if there are a large number of queries. This is why you are encouraged to name your queries : client.query(qdef, { as: 'finishedTodos'}).

The glue between the Redux store and the UI is done via ObservableQuery. ObservableQuery are objects instantiated by a component. Their role is to react to store changes and wake the component. They should not be used directly as useQuery and queryConnect do the job for you.